1. Field of the Invention
The present invention relates to a digital modulating and demodulating device for use in fixed microwave communication systems.
2. Description of the Related Art
In recent years, SDH (Synchronous Digital Hierarchy) standardized by ITU-T (International Telecommunication Union—Telecommunication standardization sector) and ITU-R (ITU—Radio communication sector) have been used as standards for communication networks for use in microwave communication systems. SDH was preceded by transmitter standards called PDH (Plesiochronous Digital Hierarchy).
Circuit arrangements of conventional digital modulating and demodulating device used in PDH and SDH systems will be described below with reference to FIGS. 1 and 2 in the accompanying drawings.
FIG. 1 shows a conventional PDH device. As shown in FIG. 1, the conventional PDH device comprises PDH line interface unit (LIU) 5, 6, signal multiplexer (MUX) 10, signal demultiplexer (DeMUX) 11, transmitter digital processing unit (TDPU) 14, receiver digital processing unit (RDPU) 15, modulator (MOD) 18, and demodulator (DEM) 19.
In FIG. 1, a number of PDH baseband signals such as E1/T1 signals applied to PDH baseband signal input terminal 1 are transmitted through PDH line interface unit 5. Then it is processed by signal multiplexer 10, transmitter digital processing unit 14, and converted into a module signal in the modulator 18. The modulated output signal is derived in the terminal 22. On the receiver side, a modulated signal applied to modulated signal input terminal 23 is processed by demodulator 19. This is then sent to the receiver digital processing unit 15, and signal demultiplexer 11 divides it into a number of baseband digital signals. The signals are then transmitted through PDH line interface unit 6 and outputted from PDH baseband signal output terminal 2.
The E1 signal represents a transmitter unit according to the European hierarchy standards and has a transmitter rate of 2.048 Mbps. The T1 signal represents a transmitter unit according to the US and Japanese hierarchy standards and has a transmitter rate of 1.544 Mbps.
FIG. 2 shows a conventional SDH device. As shown in FIG. 2, the conventional SDH device comprises SDH physical interfaces (SPI) 30, 31, SDH demapping circuit 34, SDH mapping circuit 35, signal multiplexer (MUX) 10, signal demultiplexer (DeMUX) 11, transmitter digital processing unit (TDPU) 14, receiver digital processing unit (RDPU) 15, modulator (MOD) 18, and demodulator (DEM) 19.
In FIG. 2, an SDH signal such as an STM (Synchronous Transport Module)-1 signal applied to SDH signal input terminal 26 is transmitted through SDH physical interface 30 and processed by SDH demapping circuit 34. Then it is fed to the signal multiplexer 10, transmitter digital processing unit 14 and finally into a modulated signal in the modulator 18. The output from modulated signal is derived in the output terminal 22. A modulated signal applied to modulated signal input terminal 23 is processed by demodulator 19, receiver digital processing unit 15, signal demultiplexer 11, and SDH mapping circuit 35 into an SDH signal, which is transmitted through SDH physical interface 35 and output from SDH signal output terminal 27.
STM-1 represents one of the transmitter units prescribed according to SDH, and is a signal having a transmitter rate of 155.52 Mbps.
At present, standards used in optical communication networks are shifting from PDH to SDH. However, since both PDH and SDH are currently present as standards in optical communication networks, PDH device or SDH device need to be selectively installed depending on the standards of signals to be transmitted and received. A PDH signal used in PDH device is a signal having a number of parallel channels as with E1/T1 signals, whereas an SDH signal used in SDH device is a two-wire signal having multiplexed channels such as an STM-1 signal. Therefore, PDH device and SDH device are structurally different from each other, and need to have other structural details for different transmitting directions.
As disclosed in Japanese laid-open patent publication No. 2000-224211, systems which handle both PDH and SDH signals are required to perform conversion processes such as multiplexing and demultiplexing and also to have complex connections, and need to have various system arrangements depending on the usage of signals to be handled.
FIG. 3 of the accompanying drawings shows a system arrangement of an intermediate repeater station employing the conventional PDH device shown in FIG. 1. As shown in FIG. 3, the intermediate repeater station comprises transmitter/receivers (TR) 95, 96, antennas 97, 98, and digital modulating and demodulating device 90, 91 each having the construction shown in FIG. 1.
A PDH signal such as an E1/T1 signal that is input to and output from digital modulating and demodulating device 90, 91 comprises a number of parallel channels. Therefore, two digital modulating and demodulating device 90, 91 need to be interconnected by a number of baseband signal cables. Because two digital modulating and demodulating device 90, 91 need to be interconnected by a number of baseband signal cables, the system shown in FIG. 3 suffers various difficulties such as interconnection errors, tedious and time-consuming installation processes, etc.
FIG. 4 of the accompanying drawings shows a system arrangement of an intermediate repeater station employing the conventional SDH device shown in FIG. 2. As shown in FIG. 4, the intermediate repeater station comprises transmitter/receivers (TR) 95, 96, antennas 97, 98, and digital modulating and demodulating device 92, 93 each having the construction shown in FIG. 2.
An SDH signal such as an STM-1 signal that is input to and output from digital modulating and demodulating device 92, 93 comprises a signal of multiplexed channels. Therefore, two digital modulating and demodulating device 92, 93 may be interconnected by only two coaxial cables, one for inputting a signal and one for outputting signal. However, because the plural channels are multiplexed into one signal, if a new channel is to be added to the signal between two digital modulating and demodulating device 92, 93, the system requires an ADM (Add Drop Module) device 94 as shown in FIG. 5 of the accompanying drawings.
As described above, the conventional digital modulating and demodulating device are available as structurally different PDH and SDH device, which need to have other structural details for different transmitting directions. PDH device have to be interconnected by as many cables as the number of PDH baseband signals such as E1/T1 signals, and SDH device have to have a separate device for dropping, inserting, and dividing an SDH baseband signal.